In recent years, the printing step and development processing step for photographic materials for print use have been conducted ever more quickly, and this trend requires of the materials for print use still more emphatically to have high sensitivity, to be stable in the course of photographic processing, and to be durable upon handling. The most basic method for enhancing the sensivity of a silver halide emulsion consists in increasing optical absorption per silver halide grain through an increase in grain size. When an emulsion is spectrally sensitized, on the other hand, the increase in optical absorption can be effected by increasing optical absorption on the part of sensitizing dyes which can transmit photoelectrons to silver halides to result in the formation of latent images. However, these methods do not necessarily always lead to satisfactory results. For instance, it happens that an increase in grain size hinders the speeding-up of development processing, while spectral sensitization causes inhibition of development or desilvering. In general, what is worse, it rarely occurs in spectrally sensitized emulsions that there is room for enhancing the sensitivity through further addition of sensitizing dyes or other means. Therefore, methods for enhancing the sensitivity of silver halide grains without increasing the grain size or the addition amount of sensitizing dyes are very desirable.
Representative of such methods are those known as so-called chemical sensitization, with specific examples including the method of using a sulfur sensitizer like sodium thiosulfate, the method of using a gold sensitizer like potassium chloroaurate, the method of using a reduction sensitizer like stannous chloride, or the method of using these sensitizers in combination. Although these chemical sensitizing methods control the attainable sensitivity through a proper choice of the kind and amount of sensitizer to be used, or a manner of adding and a way of combining sensitizers, it is impossible to decide the attainable sensitivity by these factors alone, for different results are brought about depending on properties possessed of silver halide grains themselves prior to an application of chemical sensitization. For example, the fact that effects produced on silver halide grains by sulfur sensitization differ in different crystal habits is described in The Journal of Photographic Science, Vol. 14, pp. 181-184 (1966). In addition, the role played by the crystal habit of grains in the latent image formation, including in the reduction sensitization step also, is described in ibid., Vol. 23, pp. 249-256 (1975). Moreover, the effects of halogen species forming grains in an emulsion and a crystal habit of the grains upon light sensitivity gained by sulfur sensitization and gold-sulfur sensitization and upon fog caused thereby are described in Photographic Science and Engineering, Vol. 28, pp. 146 to 149 (1984). However, those reports describe only the influences which the properties of silver halide grains exercise upon chemical sensitization and photographic speed, and they do not describe any art and methods which can respond to the foregoing demands in the market, that is, the attainment of further increases in sensitivity and handling stability.
Accordingly, it has been strongly desired to find out methods for achieving efficient enhancement of the sensitivity without increasing the size of silver halide grains. In addition, such methods are expected to be accompanied with further improvements on stability during the photographic processing and durability upon handling.
As a method for enhancing the sensitivity of silver halide, a grain-forming method is known as described in Japanese Patent Publication No. 36978/75, wherein so-called halogen replacement is utilized.
Silver halide emulsions prepared using such a method are characterized by increased sensitivity and reduced pressure marks. However, we have found that such emulsions have serious defects, notwithstanding the desirable characteristics as described above. Particularly, such emulsions show a decrease in mechanical pressure marks on the one hand, but on the other hand a marked desensitization phenomenon is observed in areas to which mechanical pressure has been applied prior to exposure. Although decreasing the number of halogen atoms to be replaced makes it possible to mitigate the desensitization phenomenon, such tends to increase generation of pressure marks, and what is worse, it results in both pressure marks and desensitization occurring in undesirable conditions in most cases. That is to say, reduction of pressure marks seems to be incompatible with mitigation of the desensitization phenomenon. Moreover, it has been found that reduction of halogen replacement rate brings about decrease in contrast of the emulsions.
Japanese Patent Publication No. 46932/74 and Japanese Patent Application (OPI) No. 9653/84 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") each discloses a silver halide photographic material containing a support having thereon at least one of a spectrally sensitized and monodispersed silver chlorobromide emulsion which is prepared by the steps of, in sequence, subjecting the silver halide emulsion to chemical sensitization (ripening) and then adding thereto a specific amount of a water-soluble bromide. However, these prior art photographic emulsions are insufficient for minimizing pressure marks.